93 / 2022-01-27 21:24:05

Multiple experiment-based rock typing for eolian sandstones

Eolian sandstone,rock typing,petrographic analysis,reservoir quality

A Paleozoic formation composed of various sandstones that deposited in an arid to semi-arid environment show intensive heterogeneity in terms of their sedimentary textures and reservoir quality. The variation of reservoir properties (including electrical properties) hampers accurate reservoir evaluation and petrophysical interpretation. The objective of this study is to conduct a comprehensive rock type delineation through a multidisciplinary approach, and thus provide a fundamental geological framework for quantitative petrophysical interpretation and modeling. Core descriptions were performed to determine sedimentary facies. Thin sections impregnated with blue epoxy resin were analysed using home-developed software Pore Structure Analysis Tool (PSAT) to quantify pore distribution and grain size distribution. Scanning electron microscopy (SEM) was used to distinguish clay species and distribution. Routine core examination was measured to quantify reservoir quality. X-ray diffraction (XRD) was used to analyse minerals quantitatively. Mercury injection capillary pressure analysis was undertaken to quantify pore throat distribution. By combining all these analysis, rock types were finally identified. The reservoir rocks are dominated by very fine to fine grained subarkose and quartz arenite, with less amount amounts of sublitharenite. Pore spaces are dominated by intergranular pores with rare intragranular dissolution pores. Both detrital and authigenic clays are present. In particular, samples closer to unconformity contain more infiltrated clays. Whereas, authigenic clays are often associated with partially-dissolved potassium feldspar grains or occur as pore-filling materials that contain numerous micropores. Six rock types are defined, including reworked dune sandstone (Type I), damp interdune sandstone (Type II) and dune-playa sandstone (Type III), interdune sandstone (Type IV), tidal sand flat (Type V) and pedogenetic sandstone (Type VI). Type IV rocks show the best reservoir quality with the lowest percentage of micropore, whereas Type II show the worst reservoir quality with the highest percentage of micropore. The rock types defined by this study can be used for rock type-oriented petrophysical interpretation. This study provides a novel multidisciplinary approach to delineate rock types in complex reservoirs to advance our understanding of geological controls on reservoir quality and electrical properties of the complex reservoirs, and thus provide crucial geological constrains for petrophysical interpretation and modelling.